For a display of, for example, a mobile electronic device such as a mobile phone or a mobile information terminal, a personal computer, a television receiver or the like, a monochrome or color liquid crystal display panel, an electroluminescence display panel using electroluminescence of an inorganic or organic material, a plasma display panel or the like is used.
In the case where the gray scale display capability of pixels of such a display is low, in other words, in the case where the number of gray scale levels of the display is small, a contour-like line is displayed at a border of different gradation levels of an image, which deteriorates the image quality. It is known that use of an error diffusion method improves the image quality in such a case.
According to the “error diffusion method”, an error caused when multi-value image data is converted into, for example, binary image data (i.e., difference between the multi-value image data and the binary image data) is “diffused” by multiplying a plurality of pixels adjacent to the pixel corresponding to the data by a weight coefficient. With the error diffusion method, an error caused between a multi-level original image and, for example, a binary half-tone image is minimized in an averaged manner, and thus a half-tone image having a high image quality is generated.
In the meantime, the error diffusion method has a problem that even in the case where only a part of the original image is changed, the change caused by the error diffusion is propagated over a wide range of the image, which provides a viewer with an impression that the image flickers. This problem is conspicuous in the case where, for example, a moving image is embedded in a part of the image whereas the remaining part of the image is still.